Some of the smallest lathes used by the watchmaker and driven by a single horsehair, having been noticed, a few words may be devoted to a lathe of the opposite character, fig. 92, interesting also, as having been constructed by Mr. James Nasmyth, of Patricroft, Manchester, the inventor of the steam hammer; to whose kindness the author is indebted for the following particulars.
The mandrel of this lathe measures ten and a half feet in length, and its cylindrical bearings work in brass collars the one fifteen, and the other ten inches in diameter. End play is prevented, by the mandrel being enlarged to twenty inches diameter between the bearings, and by a pair of folding wedges placed in a pocket at the back of the headstock, which serve to advance the hard steel tail pin. The diameter of the face plate is fifteen feet and its weight eleven tons, it is bored out to fit the cylindrical end of the mandrel, and so tightly, as to require forcing on by screw pressure, a single key being sufficient to retain it. The lathe serves for work of all ordinary forms, and has a break bed, the pit of which will receive wheels twenty-five feet in diameter; while to accommodate long shafts the bed is forty feet in length, and is provided with a popit head five feet high from the surface of the bed and nine feet in length of base, the center point being nine inches in diameter. The slide rest is about ten feet in length, it can be placed either parallel with the axis for turning shafts, or at right angles for surface; for the latter purpose the plan of the iron bed is that of a letter L, the cross piece being about twelve feet long. The slide rest can also be placed obliquely.
The driving gear is very appropriately contrived, there is one toothed wheel and one pinion on the principle of fig. 40; the pinion working in an internal wheel cast upon the back of the face plate, as in fig. 91. These two are respectively nine inches and thirteen feet in diameter, and of two and three quarters pitch ; this considerable difference in diameter producing a reduction of speed of nearly eighteen times, while the absence of wheels and pulley on the mandrel, allows a more massive and rigid form to be given to the headstock. The strap and speed pulleys, comprise a pair of reversed cones placed horizontally at one side, parallel with, but at a small height above the mandrel. The one A. is fixed on the pinion shaft, the other B. upon the axis carrying the driving pulley C. The speed may be varied about threefold before starting the lathe, by changing the position of the driving strap upon the fillets of the driving pulley C.; and again, about seven times when the lathe is revolving, by the traverse of the second strap along the cones A., B. The traverse of this strap is effected by a long guide screw connected with the pinion shaft, so that the revolution of the lathe is accelerated or retarded in accordance with the diameter, hence all portions of the surface being turned always pass the tool at the same rate, whatever may be its diameter and whether the tool be proceeding to or from the center of the disc.
The driving pulley C. may be made to run about eight times as fast, as the edge of the work of twenty five feet diameter, which provides ample power for that size. The quickest speed is about twenty times as fast, and therefore in proportion to work one twentieth of that diameter, or that of about fifteen inches, perhaps the smallest work that would be required in so large a lathe. The steadiness of cut was considerably improved, by an additional source of momentum in the fly wheel D., fixed upon the pinion shaft immediately behind the face plate. The fly wheel measures eight feet in diameter and alone weighs two tons.
The total weight of the lathe without its stone work is 60 tons, 16 cwt., while that of the watch maker's little turn with its centers, previously mentioned, is but 4½ oz. troy; by comparison, every grain in the watch maker's lathe represents 63 lbs. 1 oz. of its huge relative, while the watch pivot of one three hundredths of an inch diameter turned in the former, is but the 90,000th part of the circle of twenty five feet diameter that may be turned in the latter. The individual products of the two lathes also offer an excellent illustration of the wide range covered by the apparatus and the art of the turner.